1. Field of the Invention
This invention relates to a semiconductor laser apparatus attaining laser oscillation with a stabilized oscillation wavelength, which is useful in such fields as optical communication, optical measuring, optical information processing, etc.
2. Description of the Prior Art
With enlarged-applications of semiconductor lasers in such fields as optical communication, optical measuring, optical information, etc., semiconductor lasers are required to have a stabilized oscillation wavelength. The oscillation wavelength of conventional semiconductor lasers varies continuously or discontinuously with changes in temperatures and/or currents, resulting in optical output power noise. In order to solve these problems, semiconductor laser devices including diffraction grating type lasers (such as distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) lasers, etc.), internal reflector interferometric lasers, compound resonator type lasers, external resonator type lasers, etc., have been developed so as to stabilize their oscillation wavelengths. However, they have the following drawbacks: Conventional diffraction grating type lasers such as DFB lasers, DBR lasers, etc., have a diffraction grating in the waveguide, which makes the production process complicated and it is difficult to use some semiconductor materials.
Conventional internal reflector interferometric lasers, which have different effective refractivities within the resonator thereof to produce an internal reflection resulting in an interference effect by which the selectivity of a longitudinal mode can be attained, lack reproducibility and cannot achieve a strong internal reflection, making it difficult to put them into practical use.
With conventional compound resonator type lasers including cleaved coupled cavity lasers (in which two semiconductor lasers are coupled by their facets and/or which are separated into two semiconductor lasers by an etching technique), the two semiconductor lasers operate independently, resulting in the synchronization of their wavelengths, making possible the stabilization of the oscillation wavelength. However, their operation relies upon the skill of skilled workers and precise control, otherwise small changes in the spacing between the two laser devices cause changes in the longitudinal mode, resulting in optical output power noise.
One conventional external resonator type laser is shown in FIG. 2, wherein a semiconductor laser device 1 is fixed on a mounting base 2 and laser light from the light-emitting front facet of the laser device 1 is radiated outside of the laser device 1 through a window 4. The mounting base 2 is fixed on the table 5 surrounded by the window 4 and the side walls 6. An external reflector 3 is also fixed on the table 5. A part of the laser light from the light-emitting rear facet of the laser device 1 is reflected by the external reflector 3 and returns to the laser device 1.
Due to the above-mentioned structure, the external longitudinal mode (.lambda.e=2L/(me+1/2)), which is given by the distance L, between the light-emitting rear facet of the laser device 1 and the external reflector 3 occurs, so that the laser device 1 can stably oscillate only in the longitudinal mode around the peak at which the gain distribution of the internal longitudinal mode (.lambda.=2nl/m), given by the internal cavity length l of the laser device 1, is in accord with that of the external longitudinal mode (.lambda.e), wherein m and m.sub.e are an integer and n is the effective refractivity of the semiconductor laser waveguide, and nl is nearly equal to from 2L to 20L. When n is equal to 4.0 and l is equal to 250 .mu.m, the external cavity length L is set at a value in the range of 50 .mu.m to 0.5 mm. As is well known, the interval .DELTA..lambda. between the internal longitudinal modes is represented by the equation .DELTA..lambda.=.lambda..sub.0.sup.2 /2nl, and the interval .DELTA..lambda.e between the external longitudinal modes is represented by the equation .DELTA..lambda.e=.lambda..sub.0.sup.2 /2L, wherein .lambda..sub.0 is the oscillation wavelength. The longitudinal mode interval at which the internal longitudinal modes are in accord with the external longitudinal modes is in the range of 6 to 60 .ANG. at an oscillation wavelength of about 7800 .ANG., and accordingly the laser device 1 stably oscillates in a longitudinal mode when the peak of the gain distribution is in the range of said difference in the wavelength. However, it is difficult to place the semiconductor laser device 1 on the mounting base 2 such that the light-emitting rear facet faces the external reflector in a parallel manner, and moreover the production process is complicated.
FIG. 3 shows another external resonator type laser which has a semiconductor chip 7, on the facet of which a metal film 8 made of Au, etc., which is disposed by an evaporation process, instead of the external reflector 3 used in the external resonator type laser shown in FIG. 2. The reflectivity of the metal film 8 with regard to laser light has an upper limit due to light absorption of the metal, and moreover the reflectivity tends to be reduced due to oxidation of the metal. Moreover, the semiconductor chip 7 only functions as a reflecting surface.